This invention relates to a method for detecting faults in position control systems, and more specifically, for detecting faults when the actual position of a controlled state does not approach the desired position in a timely manner.
Motion control systems, which attempt to drive some movable device from some initial position to some desired position, typically have some means of determining when a fault exists in the system such that the movable device is not approaching the desired position appropriately. Generally, these systems attempt to discern between a low performance approach to the desired position and a faulty approach to the desired position. Low performance approaches may be forgiven, while faulty approaches must be diagnosed as faults and treated by some fault treatment scheme.
One such system in the prior art detects a fault when the magnitude of the difference between the desired motor position and the actual motor position exceeds a predetermined value for a predetermined time. All position errors are compared to the predetermined value and, if they exceed that value, they are given a predetermined amount of time to correct, or decrease to a magnitude smaller than the predetermined value. If they do not so correct, a fault is assumed to have occurred.
While this method, or variations of it, have been used extensively for the detection of faults in motion control systems, they have limitations. Small position errors, including those resulting from a system failure, that do not exceed the predetermined value are not detected by this system, and thus are always tolerated. This undesirable effect may be reduced by decreasing the magnitude of the predetermined value. However, such a decrease will increase the probability of assuming a fault when the system is operating normally. Such false alarms can result in the setting off of annoying alarms or can result in shutting down a "healthy" system.
Another limitation is the inability of this system to discriminate between the sources of the position error, and thus between the sources of the fault. A normal operating condition involving a large change in the position error caused by a large change in the desired motor position cannot be distinguished from an abnormal operating condition involving a large change in position error caused by a large, unexpected change in the actual motor position. In such conventional systems, conditions from either source are considered normal if they can be corrected within the predetermined time.